BOSTON (WUSA9/AP) -- Two explosions occurred in Boston, killing 3, and injuring more than 170 people, according to the Associated Press.

Fifteen of the injured are in critical condition according to hospital officials.

At approximately 2:50 p.m, the first blast occurred at the finish line on the north side of Boylston Street. This was near or possibly on the sidewalk where many spectators were standing watching runners as they made their way to the finish line. The second blast occurred 50-100 yards away, approximately 15 seconds after the first blast, police said.

There were two booms heard from near the finish line inside the Fairmount Copley Plaza Hotel. A senior U.S. intelligence official said two other explosive devices were found nearby. The official spoke on condition of anonymity because he was not authorized to discuss the findings publicly, the Associated Press reports.

A third incident at the John F. Kennedy Library was confirmed by police but it is unclear at this point whether all the attacks were related.

According to AP, some 27,000 runners took part in the 26.2-mile race, one of the world's premier marathons.

"There are people who are really, really bloody," said Laura McLean, a runner from Toronto, who was in the medical tent being treated for dehydration when she was pulled out to make room for victims of the explosions. "They were pulling them into the medical tent," said in an interview with AP.

Roupen Bastajian, a 35-year-old state trooper from Smithfield, R.I., had just finished the race. He says in an interview with AP, "We started grabbing tourniquets and started tying legs. A lot of people amputated...At least 25 to 30 people have at least one leg missing, or an ankle missing, or two legs missing."

Smoke can be rising from the blasts in photos and videos. The Federal Aviation Administration announced a temporary flight restriction over Boston.

"We are recommending people to stay indoors," Police said in a news briefing. For security purposes to not congregate in large crowds until they stabilize the situation.